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Journal of the Geological Society

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Syntectonic monazite in low-grade mudrocks: a potential geochronometer for cleavage formation?

^{1 1}British Geological Survey, Keyworth, Nottingham NG12 5GG, UK (e-mail: prwi@bgs.ac.uk)
^{2 2}Department of Geology, University of Leicester, University Road, Leicester LE1 7RH, UK
^{3 3}School of Earth and Environmental Sciences, University of Portsmouth, Burnaby Road, Portsmouth PO1 3QL, UK
^{4 4}NERC Isotope Geosciences Laboratory, BGS, Keyworth, Nottingham NG12 5GG, UK

Monazite paragenesis during low-grade metamorphism is poorly understood. The first unequivocal instance of the mineral having crystallized synchronously with compressional cleavage formation is reported. Compositionally, these monazites resemble diagenetic forms but differ texturally by displaying syntectonic microfabrics. Their predominant association with organic-walled fossils implies that organic material may play an important role in controlling the behaviour of light rare earth elements (LREE) during low-grade metamorphism, probably by releasing diagenetically sequestered LREE and organically bound phosphorus during thermal maturation (devolatilization), and/or by promoting a favourable environment for monazite precipitation. Potentially, such monazites could provide a powerful geochronometer for cleavage formation.